The present invention relates to methods and devices for checking the condition of sheathings of metal pipelines, metal cables and other metal structures, all of these being simply referred to as "pipelines" in the following, which are submerged at sea or buried in the sea bed, (hereinafter referred to simply by the adjective "submarine", as well as to monitor the effectiveness of the cathodic protection of such pipelines. Complementarily, the invention may be applied to locate such pipelines provided that they are at a different potential from that of the environment in which they are situated, as is the case if they have cathodic protection. If this is not the case, they are brought to a different potential from that of the environment, so that the invention may be applied.
The invention is applicable, for example, to metal pipelines used for conveying liquid or gaseous hydrocarbons, which are laid on the bottom or embedded in the sea bed. The cathodic protection of such pipelines against corrosion is established, on the one hand, by means of evenly spaced zinc rings which form anodes, and on the other hand by means of an electrically insulating sheathing which prevents or at least restricts electrical exchanges between the pipeline and the environment, in such manner that the anode does not play any part or a small part only, if the sheathing is in good condition.
It is clearly essential for the operation of the anodes and the condition of the sheathing to be checked periodically. In practice however, it is difficult and occasionally impossible to entrust divers--even if they are equipped with improved instruments--with the task of performing a detailed inspection of the operation of the anodes and of the condition of the sheathings, since the depth of the water, the possible embedment and the very length of the pipelines which may reach several hundred kilometers, frequently represent grave or insurmountable obstacles. The trend consequently consists in making use of manned or remotely controlled craft which render it possible to establish continuous reports rapidly.
Some methods in use at present consist, for example, in measuring the electric current flows circulating in the sea water close to the anodes and sheathings, or else in determining the local variations of electrical fields, but these methods and devices are not genuinely reliable, because the quantities measured are not sufficiently representative and thus their interpretation is inaccurate.
It is an object of the invention to minimize or eliminate these disadvantages by evaluating the evolution of the differences in electrical potential between electrodes close to the pipelines when this array of electrodes has imposed on it a translatory displacement parallel to the bottom and more particularly parallel to the pipeline.
As a matter of fact, these differences in potential derive on the one hand from the exchanges of electric currents between the sea water and the pipeline which are a function of the condition of the sheathing, and on the other hand from the exchanges between the sea water and the anodes which are a function of the operation of these latter. Consequently, if it is observed that these differences in potential remain constant or rise a little upon approaching an anode, this leads to the conclusion that this latter does not operate or operates badly; if it is observed that they rise rapidly in an area comparatively distant from an anode, this leads to the conclusion that the sheathing is in poor condition.
In practice, each potential gradient may correspond not only to the current exchange investigated, but also to the circulation of electric currents extraneous to the system in question (stray currents, earth currents, etc . . . ).
To avert evaluation errors, the effects of the two electrical fields one of which originates from the system in question (being the pipeline which is to be checked, as it happens), and the other from random sources, should consequently be separated. This is why the invention is based on the application of differential measurements intended to eliminate any signal generated by an electrical field extraneous to the system in question.